docs: add 'How to add a provider' guide and link it in README

This PR adds a comprehensive “How to add a provider” guide and links it from the main README.

Requested by: Alec Fong (@theFong)
Link to Devin run: https://app.devin.ai/sessions/0459b0ec7cc14278a6a5244af378a333

Summary of changes:

• Add docs/how-to-add-a-provider.md
  • Step-by-step instructions to implement a new provider
  • Canonical references to Lambda Labs provider (client, instance, capabilities)
  • Links to key docs: docs/ARCHITECTURE.md, docs/SECURITY.md, docs/VALIDATION_TESTING.md, pkg/v1/*.go
  • Copy/paste scaffolds for credentials, client, capabilities, and instance ops
• Update README.md
  • Add a prominent link to the new guide in the Documentation section

Verification:

• make lint: passed
• make test: passed

Notes:

• Docs-only changes; no provider code modified.
• CI should pass without provider credentials (validation tests are skipped under -short in default test).